Abradable protective coating for compressor casings



June 4, 1963 v. K. EDER 3,092,306

ABRADABLE Pao'rc'rvs comme FOR COMPRESSOR cAsINcs Original Filed April 28, 1958 'INVENTUA d?? /y/Y 1 der T'RNEY United States Patent O 3,092,306 ABRADABLE PROTECTIVE COATING FOR COMPRESSOR CASINGS Virgil K. Eder, Indianapolis, Ind., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Original application Apr. 28, 1958, Ser. No. 731,435, now Patent No. 3,010,843, dated Nov. 28, 1961. Divided and this application Jan. 27, 1961, Ser. No. 85,266

3 Claims. (Cl. 230-133) This invention relates to improvements in compressors and more particularly to improvements in the eiciency of axial-flow compressors by *application of a suitable Aresin impregnated metal coating to the inner wall surface of the compressor housing. This invention is a division of applicants :zo-pending application Serial No. 731,435 filed April 28, 1958, now Patent No. 3,010,843.

Typical axial-flow compressors, such as those used in modern turbine engines, include `a rotor which carries rows of outwardly extending rotor blades or vanes and a compressor housing which carries correspon-ding rows of stator vanes. The body of the rotor may be formed of a suitable high strength forgeable material such as an alloy of titanium, aluminum, and/or steel while the rotor vanes, generally dovetailed into the rotor, may :be of stalinless steel or the like.

The compressor housing or casing, which may be cast from stainless steel, aluminum, magnesium, or the like, and various alloys thereof, generally is a two-piece assembly split on a plane through the compressor axis. Secured to the inner circumference of the housing are longitudinally spaced rows of stator vancs of stainless steel or the like which project inwardly between the corresponding rows of rotor vanes.

Up to the present time, one of the primary diiculties with axial-flow compressors has been the excessive energy loss due to the leakage of air past the rotor blade tips. The amount of air leakage depends to a large degree upon the clearance between the tips of the rotor blades and the compressor housing. This clearance in turn depends on the rigidity and dimensional stability of the compressor. In addition to the warpage and elastic deformation encountered in operation, the differential expansion of the compressor parts over the wide range of temperatures encountered in use, makes it highly impractical to manufacture a compressor having `a minimum clearance for optimum eiciency. Not only would more costly finishing and inspection operations be required 'in manufacture, but in many cases the dimensional instability of the closely fitting parts would result in damage to the compressor by `scoring r gouging of the housing or breakage of the rotor vanes.

Accordingly, the principal object of this invention is to provide effective means for minimizing the clearance between the rotor blade tips and the compressor housing and for minimizing corrosion of the housing. A further object is to provide a method for applying -a readily machinable heat-resistant coating to the interior of the compresser housing to reduce the clearance between the blade tips and the housing. A still further object is to provide a compressor having an improved operating Veiiciency by coating the interior of the compressor housing with a heat-resistant material adapted to `be machined by the rotating blade tips in establishing a, minimum operating clearance. Other objects and advantages will more fully appear from the description which follows.

I have discovered that the clearance 'between the blade tips and the compressor housing in an axial-flow compressor can be effectively reduced to a minimum by providing the interior of the compressor housing with a coating of a relatively soft metal which is `adapted to be cut away 3,092,306 Patented June 4, 1963 "ice by the tips of the moving blades to provide a minimum clearance for optimum operating efficiency, the method of application of said material to said housing being to apply a machinable porous metal coating and a resinous plastic coating to lill the pores to produce a tightly adherent, non-porous, substantially warp and crack resistant coating which is readily machinable and protects the housing from corrosion.

The invention will be best understood in connection with the accompanying drawing, in which:

FIG. l is a fragmentary view, partially in section, of a multistage axialdiow air compressor incorporating the invention; and

FIG. 2 is la fragmentary view, partially in section, of one :stage of an air compressor taken on line 2 2 of FIG. l.

Referring now to the drawing in detail, a turbo-compressor indicated generally at 1, is shown in a housing 3*, only so much of the compressor being shown as is necessary to illustrate the invention. The rotor is fabricated to carry a plurality of rows of rotor blades 5, the blades being supported on the rotor shaft in any manner well known in the art and not shown. Extending inwardly from the housing 3 between the rows of rotor blades 5 are the rows of stator vanes 7. The stator vane rows are supported by rings 9 which are suitably secured in annular grooves 11 in the housing 3.

The present invention is directed 'so means for reducing the clearance between the tips 13 of .the rotor blades S and the inner circumferential surface 15 of the compres sor housing. In accordance with this invention the portions of the housing adjacent the paths described by the tips 13 of the moving blades 5 `are provided with a porous coating 17 of a relatively soft metal such as aluminum, bronze or Zinc which is adapted to be readily machined by the blade tips to establish a minimum operating clearance between the blade tips and the rotor housing. As indicated in the fragmentary end view in FIG. 2, the coating 17 extends around the entire circumfeerntial portion of the housing adjacent the moving rotor blade tips. Also, as is described more fully hereinafter, the surface of the housing 3 which is coated is roughened as by cutting shallow `threads 19 therein as shown in FIG. 1.

I have found that by impregnating the porous coating 17 with a heat-resistant resinous plastic not only are the pores sealed thus precluding corrosion of the housing, but that the machinability of the resultant coating is very greatly improved as is also the surface smoothness and uniformity. Extended development and tests demonstrated that a metal coating having `the desired properties noted above and which is adherent to the base metal and possesses a uniform surface free from blisters, chips or other objectionable defects may be obtained `by the hereinafter described methods.

It is essential that the surface to be coated be properly cleaned and conditioned prior to application of the metal coating. The surface should be thoroughly cleaned to remove dirt, grit, and oils by solvent cleaning as by vapor degreasing in trichlorocthylene or by dip-washing in a petroleum solvent. The surface is then conditioned to enable a tightly adherent coating by machining a shallow thread in the surface and/or grit blasting or shotting to provide a roughened surface. The resultant aluminum, magnesium or steel surface should have a rather fine texture and should be substantially uniform in quality.

The porous metal coating is then applied to the desired thickness in any suitable manner Vas by spray coating with a metallizing spray gun. I have found it necessary to preheat the surface to be coated to a temperature of from about l-200 F. During the spraying operation I have found it necessary to maintain the temperature of the part between 15G-200 F. Parts deviating from these temperatures are apt to have an inferior bond between the coating metal and the base metal. The desired temperature control may be conveniently achieved by applying a hot air blast to the opposite side ofthe part being treated.

In applying a porous bronze coating to a magnesium compressor housing surface, I found that a coating thickness of from 0.003 to 0.005 inch without any intermediate bonding layer was sufficient to produce the desired characteristics. In applying a porous aluminum coating to a steel compressor housing surface it was found that an intermediate bonding layer of stainless steel produced the best results. My preferred method is to spray coat a layer of `stainless steel of from about 0.002 to 0.005 inch thickness foilowed by the application of a porous aluminum coat of from about 0.030 to 0.035 inch thickness. It should be noted that other intermediate layer materials may be utilized, i.e., bronze, zinc, copper.

While the porous metal coating is machinable, I have found that the impregnation thereof `greatly improves the machinability while at the same time producing a uniformly smooth surface and precluding corrosion of the base metal. Impregnation is `accomplished by' using a high temperature resistant thermosetting resin such as any of the well known epoxy or silicone resins in suflicient solvent or thinner to produce a solution of the desired viscosity. About equal parts by volume of epoxy resin and thinner has been 'found to be suitable. Similarly, a mixture of about 1S.0-22.0 parts by weight silicone resin solids and about 78.0-820 parts solvent is satisfactory. I have found that a minimum of two coats of impregnant should be applied in any suitab-le manner, as by brushing, spraying, dipping or rolling. Complete impregnation is accomplished when resin is still on the surface after about 20 minutes of air drying after application, this dry period being used after each coat.

After complete impregnation, the part is subjected to baking at elevated temperatures in progressive steps in order to thoroughly dry and cure the materials. I have found it to be satisfactory if the impregnated coating is first baked. at about 150 F. for at least one-half hour followed by a bake at about 450 F. `for one hour. Where the porous metal coating is relatively thick, as in 625 F. for a period of about 3 hours. Upon cooling the case of aluminum, a final bake is desirable at about to room temperature, sanding with steel wool or other finishing may be used to obtain a smoother finish, if desired.

While I have described my invention in terms. of applying the coating directly to the housing of the compressor, it should be understood that it may be `applied to parts positioned on the housing. Other embodiments may be apparent to tho-se Skilled in the `art and such embodiments are within the scope of my invention as defined by the claims which follow.

What is claimed is:

l. A compressor housing adapted to be positioned about a rotor and having a readily machinable coating on the inner surface thereof, said housing comprising a metal annular member having a porous sprayed metal coating on the inner surface thereof, the pores of said coating being filled with a high temperature thermosetting resin to seal the poires in said sprayed metal coating.

2. In a device `as set forth in claim l, ysaid coating being formed of bronze and having a thickness of from about 0.003 to 0.005 inch.

3. In a device as set forth in claim 1, said coating being formed of an intermediate bonding layer of stainless steel of a thickness of `about 0.002 to 0.005 inch upon which there is positioned a layer of 0.030 to 0.035 inch aluminum.

References Cited in the tile of this patent UNITED STATES PATENTS 899,319 Parsons et al. Sept. 22, 1908 2,217,719 Williams Oct. l5, 1940 2,720,356 Erwin Oct. 11, 1955 2,798,509 Bergquist July 9, 1957 2,930,521 Koehring Mar. 29, 1960 2,946,609 Comery July 26, 1960 2,963,307 Bobo Dec. 6, 1960 2,964,339 Macks Dec. 13, 1960 FOREIGN PATENTS 791,568 Great Britain Mar. 5, 1958 793,886 Great Britain Apr. 23, 1958 1,145,388 France May 6, 1957 

1. A COMPRESSOR HOUSING ADAPTED TO BE POSITIONED ABOUT A ROTOR AND HAVING A READILY MACHINABLE COATING ON THE INNER SURFACE THEREOF, SAID HOUSING COMPRISING A METAL ANNULAR MEMBER HAVING A POROUS SPRAYED METAL COATING 